An automatic transmission for a vehicle that controls a gear ratio on the basis of a vehicle speed, an accelerator operation amount (i.e. request drive amount), and the like is configured to set or change operational states such as a transmission path of torque and a transmission torque capacity by hydraulic pressure. For example, a stepped transmission is configured to engage or disengage a clutch or a brake by the hydraulic pressure to set a prescribed gear ratio and set the transmission torque capacity according to the hydraulic pressure. Further, a continuously variable transmission of a belt-type, a toroidal-type, or the like is configured to generate squeezing force on a transmission member such as a belt or a power roller by the hydraulic pressure to secure a prescribed transmission torque capacity and engage or disengage the clutch or the brake to perform switch between a forward travel state and a reverse travel state. In addition, a transmission has been known which is configured such that a gear pair is provided for each gear ratio, the gear pair that is involved in the torque transmission is selected from the gear pairs to set a prescribed gear ratio, the selection of the gear pair is performed by a hydraulic actuator, and input of torque to the gear pairs is performed via the clutch that is engaged by the hydraulic pressure.
An example of a belt-type continuously variable transmission and its hydraulic pressure control device among such kinds of transmissions is disclosed in WO 2010/21218. In the belt-type continuously variable transmission, a primary pulley and a secondary pulley on which a belt is wound are configured with a fixed sheave and a movable sheave so as to be capable of changing a groove width of a belt-winding groove, and a supply valve and a discharge valve that are configured with solenoid valves are connected to a hydraulic pressure chamber in each of the pulleys to change the gear ratio by supplying hydraulic pressure of a hydraulic pressure source to the hydraulic pressure chamber or to set a belt squeezing force to pressure according to the request drive amount. Further, the belt-type continuously variable transmission disclosed in WO 2010/21218 is installed in a vehicle that is configured to stop an engine when the vehicle stops and to restart the engine when the vehicle restarts traveling. Accordingly, because a mechanical type oil pump that is driven by the engine stops when the vehicle stops and does not generate the hydraulic pressure, the continuously variable transmission is provided with an accumulator and an electric oil pump that is driven by a motor to generate the hydraulic pressure. In addition, when the gear ratio or the squeezing force is maintained constant, the continuously variable transmission is configured to perform so-called zero-leakage control in which each valve is closed to shut in the hydraulic pressure. Accordingly, a valve such as a poppet valve that causes little leakage of the hydraulic pressure in a valve-closed state is employed as each valve.
Incidentally, because there have been demands for an improvement in fuel efficiency of the vehicle and reduction in exhaust gasses, so-called stop and start control (hereinafter referred to as S&S control) has recently been performed in which the engine is stopped when the vehicle stops or a prescribed condition is satisfied. When the S&S control is performed in the vehicle in which the transmission that is configured to obtain the hydraulic pressure by driving the oil pump by the engine is installed, the transmission can be controlled by the hydraulic pressure accumulated in the accumulator. Therefore, it is considered that no particular trouble occurs in a restart of travel and travel of the vehicle.
However, because the control of the transmission is performed along with supply and discharge of the hydraulic pressure and a hydraulic device has inevitable leakage of the hydraulic pressure, the hydraulic pressure gradually decreases when only the accumulator is used as the hydraulic pressure source. When the hydraulic pressure of the accumulator decreases to a predetermined reference pressure while the S&S control is performed, the engine is restarted to secure the hydraulic pressure. Thus, there is a case where the hydraulic pressure accumulated in the accumulator is low when the engine is stopped by the S&S control. In such a case, the engine is restarted due to the decrease in the hydraulic pressure in the accumulator, and a stop period of the engine may thus become short.